1. Field of the Invention
This invention relates to an apparatus and method for dissolving xe2x80x9cbiscuitsxe2x80x9d or xe2x80x9ctabletsxe2x80x9d or xe2x80x9cpucksxe2x80x9d containing chemicals into a liquid solution and more particularly a method of precisely controlling the dissolution rate of water purification tablets into solution. In addition, the invention provides for a system and method for either continuous or intermittent dispensing of the dissolved chemical into a flowing line, either pressurized or unpressurized, in a controlled manner for generating a specific concentration of dissolved chemical in water, and using the chemical solution to maintain an overall residual level of the dissolved chemical in the flowing line.
2. Description of Prior Art
Prior arrangements used to dissolve solid chemical tablets into a liquid solution are based upon the principle of liquid dissolution or physical erosion in order to break the solid tablets so that the chemical of the tablets is dissolved into solution. Most forms of solid chemical tablets are pressed into geometric shapes such as various size tablets of rectangular or cubical forms, which are bound together by using a combination of various fillers and binders. It has been necessary to sometimes use a combination of physical erosion and liquid dissolution to accomplish the dissolving process.
A characteristic of chemical tablets is an inherent inconsistency in chemical strength, because during manufacture of the chemical tablets, a mixture is first produced of dry granulated chemicals, which may contain various levels of inert fillers and binders. The dry mixture is then mixed with liquid to form a chemical mixture having a xe2x80x9cputtyxe2x80x9d-like consistency. The chemical mixture is then pressed into various shapes. Since combinations of dry and liquid products are difficult to blend evenly prior to the pressing and forming process, the final tablet often varies in consistency and strength from batch to batch or even tablet to tablet. Additionally, temperature, age, relative humidity and level of pressing pressure all affect the density, solubility and final chemical assay strength of each individual solid geometric tablet. This inherent inconsistency of the dissolution characteristics of chemical tablets such as containing calcium hypochlorite makes precise and even dissolution difficult whether the dissolution process is made a xe2x80x9cbatchxe2x80x9d at a time, or constantly as in the case of a continuous feed process. Because chemicals such as calcium hypochlorite, which when dissolved produce chlorine in the water, are often used to achieve and maintain minute levels of residual chlorine strength with a given process, (for example, water purification), and since chlorine demands within the various processes often vary, it is extremely difficult to maintain consistent performance with existing erosion dissolution apparatus and methods.
Previous equipment designed to dissolve or erode solid chemical tablets typically employ a combination of (1) variable flow rates of water across the tablets and (2) variable area exposure of the tablets to the water. U.S. Pat. No. 5,427,748, shows a chlorinator which uses a variable flow-rate of water, which correspondingly raises the level of water within the chlorinator and therefore exposes more of the surface area of the tablets in order to dissolve more chemical such as calcium hypochlorite. This method passes a variable volume of untreated water through the chlorinator in order to dissolve the desired amount of chlorine into solution which is then discharged by gravity either into an open process tank or is placed into a solution tank where it is mixed with additional untreated water to form a final solution prior to being pumped into a pressurized process line. Untreated water is passed through the feeder only one time, with no recirculation of treated water across the chemical tablets within the process. The gravity chlorinator of U.S. Pat. No. 5,427,748 delivers more or less chemical per unit time by adjusting the volume of liquid passing through the unit which correspondingly raises or lowers the water level within the chlorinator and therefore causes the water to contact more or less surface area of the chemical tablets. When the system is inactive, water drains by gravity from the feeder, leaving the tablets free of water contact.
A problem exists with the method and apparatus of dissolution of U.S. Pat. No. 5,427,748 in that because tablets are placed randomly into the feeder column of the chlorinator, the geometric shape of the chemical tablets relative to the direction of water flow as it passes up through the chlorinator produces varying degrees of dissolution. Water contacting a tablet at a perpendicular angle has more eroding capacity than the same volume of water contacting the side of the tablet at a very slight angle. Because the tablets are fed by gravity as the tablets within the flowing water are dissolved, the random position of the tablets within the stored column are randomly oriented in the feeder and are in a constant state of change, therefore producing inconsistent rates of erosion and dissolution as water flows over them. Because of the variation in erosion rates, the water flow rate may require constant adjustment through the feeder in order to maintain consistent solution strength.
Another problem with the method and apparatus of U.S. Pat. No. 5,427,748 is that with very low flow rates and erosion rates, the tablets tend to bridge. Bridging is a condition where the chemicals and fillers are eroded away while leaving a shell of binder and other solids. The remaining shell in one or more tablets tends to create a xe2x80x9cbridgexe2x80x9d that prevents the upper undesolved tablets, which are supported from the bridge, from migrating or falling into the dissolving water stream. This phenomenon reduces the amount of actual chemical dissolution over a period of time, therefore making the treated solution vary in dissolved chemical strength. A chemical such as calcium hypochlorite which is used to disinfect water is typically injected at very low concentrations. Consistent dissolution is critical. As an example, in potable water treatment, final concentrations in the process line are maintained at levels between one (1) and two (2) parts per million. Often, specifications call for fractional parts per million, such as 1.5 or 1.6 ppm making it more difficult to maintain desired levels when dissolution is not consistent.
Varying the flow rate of water through the U.S. Pat. No. 5,427,748 chlorinator controls the rate of dissolution. To make changes in flow rate requires the operation of a flow control valve either manually or by some automatic means such as a motorized proportioning valve. Since the dissolving water is xe2x80x9csingle pass-throughxe2x80x9d, the resulting output of the system must also be altered with each change in chemical demand. Since volumes of water created through the process are typically large, centrifugal pumps are normally chosen for the injection process. Centrifugal pumps are sized to rather narrow ranges of flow performance at specific pressures, making varying their output difficult. These changes can be complicated when a pump is utilized to inject the dissolved solution into a pressurized line. Due to inconsistencies in the chemical concentration of the tablets, the physical characteristics of the tablets, and of the process as described above, changes in chemical demand by the process requires perpetual adjustments of the flow rate of the system.
Altering the volume of flow through a system is often difficult, since in many cases the volume available to the feeder is fixed.
This invention has particular application in the area of liquid treatment, especially water, where disinfection chemicals including chlorine bearing chemicals such as calcium hypochlorite, di-chlorisocyanurate, tri-chlorisocyanurate, or bromine bearing chemicals, and also chemicals used for the removal of chlorine or bromine and various other products used within the water treatment industry, must be introduced in order to disinfect or otherwise treat the water for either consumption or discharge after use. Such processes are used to treat drinking water, water for swimming pools, water for cooling towers, wastewater, and sewage. Within this application, chlorine must be maintained in solution at fractional levels from one-half (0.5) parts per million to solution strengths into the single digit concentration levels such as 1.0-5.0 percent concentration (e.g. 10,000 to 50,000 parts per million). The invention provides the unique capability of producing any concentration level from a high volume-low concentration solution to a low volume-high concentration solution.
The present invention is embodied in an apparatus and method for precisely controlling the dissolution of solid chemical tablets and preparing the resulting solution for injection into a process stream. The process of dissolving solid chemical tablets is accomplished by passing a fixed rate of dissolving fluid such as water through the feeder. With a recirculating stirring action of the dissolving fluid through the feeder, the rate of dissolution can be varied and precisely controlled without varying the total volume of fluid passing through the feeder. The recirculation and mixing action is accomplished through one of several alternative arrangements and methods.